The invention relates to a retarder system.
Hydrodynamic retarders have become known, for example, from the German publication. Dubbel, xe2x80x9cTaschenbuch fxc3xcr den Maschinenbauxe2x80x9d, 18th edition, pages R49-R53. Those retarders are connected and disconnected by filling and emptying with an operating fluid a bladed working cycle.
Mainly commercial vehicles have a hydrodynamic retarder with a retarder coolant circuit. Even though said retarder coolant circuit is designed separate from the transmission oil circuit proper, it uses as a rule the transmission oil from the transmission sump as operating fluid.
Retarders are often used as wear-resistant continuously operating brakes which are provided with a rotor unit connectable with the transmission of the vehicle and for decelerating the vehicle converts the mechanical energy of the transmission to thermal energy. It is, therefore, obvious to use the transmission oil as operating fluid of the retarder.
In DE 197 51 776 A1 and DE 196 46 598 A1, for example, hydrodynamic retarders of that kind have been disclosed in which the rotor converts the mechanical energy to the energy of a fluid which for its part is changed to heat in the rotor unit whereby the desired braking power is produced.
Hydrodynamic retarders can be either coupled to the transmission to be decelerated or already integrated in it. In a common arrangement of the hydrodynamic retarder this is integrated in a transmission, preferably of commercial vehicles, and driven by a drive shaft of the transmission, the operating fluid constituting, as already mentioned, oil from a transmission sump which simultaneously feeds a separate transmission oil circuit.
In construction and operation a retarder is similar to a hydrodynamic converter which is used in opposite working direction. The hydrodynamic retarders, already known, are usually designed with a rotor unit and a stator unit, the rotor unit being designed with a rotor shaft and a rotor impeller which interacts with a stator impeller disposed spaced from the rotor impeller.
The rotor unit of the retarder is here connected with the drive shaft of the transmission while the stator unit is fixedly situated on a retarder or transmission housing. As a consequence of the loading with a torque from the rotor unit, the operating fluid located between rotor impeller and stator impeller is driven and circulated by the rotor unit, similarly as in a pump, in the working cycle of the retarder. The hydraulic flow resistance of operation fluid is braking the output shaft of the transmission, a certain differential rotational speed having always to be maintained.
The kinetic energy consumed as result of the delay or of the deceleration of the rotor results in a heating of the transmission oil. To prevent an overheating respectively of the retarder or of the oil, the heated transmission oil has to be cooled off in an adequate manner when passing through a radiator.
To eliminate the heat produced in the retarder, a retarder coolant circuit is usually provided which comprises the retarder with the stator unit and the rotor unit thereof and the radiator. In the activated state of the retarder, said retarder coolant circuit is closed as intrinsic working cycle of the retarder, there being conveyed, via a control valve and a retarder pump, operating fluid which has been lost due to inner leakage in order to maintain the filling level in the system. In a deactivated state of the retarder, the transmission oil is delivered through the radiator by the retarder pump according to the rotational speed of the transmission and thus to the velocity of the vehicle while circumventing the rotor and stator units of the retarder, and then returned to the transmission sump. The radiator in the coolant circuit of the retarder is a heat exchanger known per se which can communicate with a cooling water system of the motor.
At a low speed of the vehicle, a correspondingly small amount of transmission oil is passed through the heat exchanger. Since the degree of cooling power essentially depends on the flow rate, the cooling of the transmission oil is in those cases rather ineffective and the radiator is marked by a poor degree of efficiency.
In addition in certain driving situations, there exist specific load conditions which require an increased flow rate of transmission oil through the radiator. Here is to be mentioned such a driving situation in which a high working capacity with simultaneous low velocity occurs as is the case, for example, in mountain driving of a commercial vehicle under full load. It is problematic here that in accordance with the low velocity of the vehicle, the transmission pump makes available to the radiator only a correspondingly low amount of oil and the cooling power needed for this case of load can only be attained by a considerably larger amount of oil. The degree of efficiency of the radiator is also low in this case.
The problem on which this invention is based is to provide a retarder system in which with simple means the flow rate of transmission oil through a radiator of a retarder can be optionally increased in certain driving situations that occur for the purpose of obtaining better cooling power.
The heat exchanger of the retarder coolant circuit is basically laid out for a multiplicity of flow rates of transmission oil that the retarder pump can make available, as this must always be the case, for example, for the braking state of the retarder, that is, in its closed working cycle. Consequently an exiting amount of transmission oil exceeding the amount normally flowing through the radiator depending on the prevalent velocity can always be easily absorbed by the radiator.
According to the invention the flow rate can be optionally increased without this exclusively depending on the actual velocity of the vehicle. According to the invention this is possible for a low velocity of the vehicle both under normal load and under full load or overload like, for example, in a mountain drive.
The better cooling power that goes along with an increased flow rate lowers the maximum temperature of the transmission oil occurring during the cooling operation whereby the service life of this operating fluid is decisively extended. Besides, the degree of efficiency of the heat exchanger used increases. With an altogether better cooling behavior is also associated a subsequent re-regulation of the retarder.